Slaviansk: Ukrainian forces moved against pro-Russian forces manning checkpoints outside the eastern Ukraine city of Slaviansk, killing and wounding a still undetermined number of people and prompting Russia to launch what the country’s defence minister said were military exercises along the Ukrainian border.

The moves sharply raised tensions in the developing crisis. In Moscow, Russian President Vladimir Putin said any fighting would have an impact on Ukraine’s relations with Russia and would prove that Russia was justified in interfering in Crimea.

Ukrainian security force officers at a checkpoint set on fire and left by pro-Russian separatists near Slaviansk. Photo: Reuters

“If, in fact, the Kiev regime has started to use the armed forces against people inside the country, then, with no doubt, it is a serious crime against their own nation,” Mr Putin said at a forum in St Petersburg for regional reporters and media figures that was broadcast live on Rossiya 24 television.

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The Russian military exercises seemed likely to further fray relations with the United States and its Western allies, who have demanded repeatedly that Russia cease its efforts to stir unrest in eastern Ukraine and desist from military action along the border, where the Kremlin has massed as many as 40,000 troops.

Russian Defence Minister Sergei Shoigu said the exercises would include troop movements on the ground as well as flights by the Russian air force. Mr Shoigu also complained about NATO exercises in Poland and the Baltics, which the alliance announced recently in response to previous Russian threats of military intervention in Ukraine.

American journalist Simon Ostrovsky has been released in eastern Ukraine. Photo: AP

“The starting gun on the use of weapons against their own civilians has already been fired,” Mr Shoigu said, according to the Interfax news service. “If today this military machine is not stopped, it will lead to a large number of dead and wounded. We have to react to such developments.”

Ukraine's acting President Oleksandr Turchynov said his country will not yield to what he called a "terrorist threat" and told Russia to "stop interfering".

"We will not back down from the terrorist threat," he said in a televised address.

Ukrainian troops move into a new position a few kilometres from Slaviansk. Photo: Reuters

Washington criticised the Russian drills on the frontier. It was "exactly the opposite of what we have been calling on the Russians to do, which is to de-escalate the situation," Pentagon spokesman Colonel Steve Warren said.

Ukrainian forces were reported to have engaged pro-Russian separatists in the eastern part of the country in what appeared to be a limited action. There were no confirmed reports of casualties, but the Ukrainian government said five separatists had been killed while several people claiming to have witnessed the fighting put the number at one to three. The Ukrainian forces appeared largely unscathed.

Russia on Thursday demanded the US force the Ukrainian authorities to halt the military operation in south-eastern Ukraine and withdraw units of the armed forces. "We are counting on the United States to take urgent measures in the interests of de-escalation," it said in a statement.

Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry were scheduled to speak via telephone on Friday.

In his most detailed accusation of Russian interference to date, Mr Kerry said that US intelligence services had concluded that Russia's "military intelligence services and special operators are playing an active role in destabilising eastern Ukraine".

"Some of the individual special operations personnel who were active on Russia's behalf in Chechnya, Georgia and Crimea have been photographed in Slaviansk, Donetsk and Luhansk," Mr Kerry said. "Some are even bragging about it by themselves on their Russian social media sites."

Vyachislav Ponomaryov, the de facto mayor of Slaviansk, said on Tuesday that armed men had come to his town from outside Ukraine but insisted they were friends and volunteers, not Russian special forces.

American journalist Simon Ostrovsky has been released in eastern Ukraine, according to online news site Vice News, for whom he was working in Slaviansk when he was held by pro-Russian separatists on Monday.

Ukrainian interim authorities said “civilian activists” had regained control of city hall in the south-eastern city of Mariupol on the Sea of Azov, forcing pro-Russian protesters to leave without bloodshed. There was no independent corroboration of the account, published by Interior Minister Arsen Avakov on Facebook. News reports offered a different version of the events, saying the building had been stormed by masked men who used baseball bats to beat the occupiers.

Referring to the interim leadership in Kiev as a “junta", the usual description used by the Russian government, Mr Putin said Ukraine’s decision to act was “just a punitive operation” that would have consequences, including on “intergovernmental relations” between Moscow and Kiev.

Russia maintains it was forced to intervene in Crimea, and ultimately to annex the peninsula, because of a threat to the safety of Russians living there. No such incidents were confirmed by outside observers, but on Thursday Mr Putin said: “What we can see in Ukraine’s east, undoubtedly, would have happened in Crimea, had we not taken certain timely measures to protect the interests of the people in Crimea.”